1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a method for restoring a liquid ejection head, and more particularly, relates to technology for determining the ejection state of liquid from a liquid ejection head of a liquid ejection apparatus, and to a method for restoring the liquid ejection head in cases where an ejection failure is detected.
2. Description of the Related Art
As an image forming apparatus, an inkjet recording apparatus (inkjet printer) is known, which comprises an inkjet head (ink ejection head) having an arrangement of a plurality of nozzles (ink ejection ports) and which records images on a recording medium by ejecting ink (ink droplets) from the nozzles toward the recording medium while the inkjet head and the recording medium are caused to move relatively to each other.
Various methods are known as ink ejection methods for the inkjet recording apparatus of this kind. For example, one known method is a piezoelectric method, where the volume of a pressure chamber (ink pressurization chamber) is changed by causing a diaphragm forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element, ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being ejected as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method where ink is heated to generate a bubble in the ink, and the ink is then ejected by means of the expansive energy created as the bubble grows.
In an image forming apparatus having an ink ejection head such as an inkjet recording apparatus, ink is supplied to the ink ejection head via an ink supply channel from an ink tank which stores ink, and this ink is ejected by one of the various ejection methods described above. However, it is necessary that ink is ejected stably in such a manner that factors, such as the ink ejection volume, the ejection velocity, the ejection direction, and the shape of the ejected ink (presence or absence of satellite droplets and volume of ink), conform to uniform values at all times.
However, during printing, the nozzles of the ink ejection head are filled with ink at all times, in order that printing can be performed as soon as a printing instruction is issued. Therefore, the ink in the nozzles is exposed to the air, and the ink in nozzles that do not perform ejection for a long period of time dries, the viscosity of the ink increases, it becomes impossible to eject ink droplets satisfactorily, and nozzle blockages may occur, leading to ejection failures. Furthermore, if the ink supply is interrupted because there is stagnation of air bubbles mixed into the ink supply channels, or the like, or if an ejection operation is continued for a long period of time, then ink refilling may be delayed, leading to ejection failures.
Due to reasons such as these, it is necessary to perform maintenance of the ejection head when an ejection failure has occurred or ink is no longer being ejected in a stable fashion as described above. Therefore, various methods have been proposed in order to determine whether ink is being ejected stably or not, and whether the ejection head has produced ejection failures or not.
For example, a method is known in which the quantity of electrical charge flowing to the electrode of a piezoelectric element which pressurizes ink in an ink pressurization chamber is measured, the ink pressure inside the ink pressurization chamber is determined on the basis of the measured quantity of electrical charge, and problems occurring in the ink flow channels, such as the presence of air bubbles in the ink flow channels, or increased fluid resistance of the ink flow channels caused by dirt, are detected on the basis of this pressure (see, for example, Japanese Patent Application Publication No.11-99646). Furthermore, in this determination process, a threshold value is established with respect to the peak value, and an abnormality is taken to have occurred if this threshold value is exceeded. For example, if the pressure value exceeds the peak value of the positive pressure during normal operation by +10%, then an abnormal increase in fluid resistance due to dirt is deduced, and if the pressure value does not reach a value of −10% of the peak value of the positive pressure during normal operation, then stagnation of air bubbles is deduced.
Furthermore, for example, a method is known in which the pressure wave created inside a pressure chamber as a result of applying a measurement voltage waveform is determined, and a drive voltage waveform suited to the characteristics of that pressure wave is calculated. Therefore, the drive waveform is controlled on the basis of the use environment, such as the temperature, and the properties of the ejection liquid, and the like, in such a manner that liquid droplets having a prescribed speed of flight and a prescribed droplet volume are ejected at all times, without any timing delay between the movement of the piezoelectric element and the pressure change, even if there is variation in the period of the pressure wave or the attenuation rate. Accordingly, variations in ejection are suppressed (see, for example, Japanese Patent Application Publication No. 7-132592).
Furthermore, for example, a method is known in which, when performing experimental ink ejection for determining the ink ejection state, the pulse width of a signal applied to a recording head is set to the minimum pulse width which allows ink to be ejected stably when the pulse signal is applied to the recording head, and hence the noise incorporated into the electrical circuitry of the determination system (electromagnetic induction noise) is reduced and stable determination can be performed (see, for example, Japanese Patent No. 3495898).
If the pulse width is set as described above, then in a recording head which ejects ink by means of thermal energy, adequate ejection conditions are not obtained in any heaters which are starting to produce deterioration in ejection, and hence an ejection failure will be detectable, thus making it possible to prevent sudden occurrence of ejection failures during image printing. Furthermore, here, the actual process of determining ejection failures is performed by means of a photosensor, such as a transmissive type photo-interrupter, or the like, which determines, outside the head, the ink droplets ejected from the nozzles by means of a direct optical method.
However, in the apparatus described in Japanese Patent Application Publication No. 11-996646, ejection failures are determined by measuring the quantity of electrical charge flowing to the drive piezoelectric element and determining the ink pressure inside the ink pressurization chamber, but there is no particular disclosure with regard to the design of the voltage waveform applied to the drive piezoelectric element when an ejection failure is determined. Moreover, Japanese Patent Application Publication No. 11-99646 indicates waveform data in the case of air bubbles that are several tens of times the size of the ejection ink droplets, or waveform data in the case of a state where the nozzles are completely blocked. However, usually, ejection failures are a problem even when there are air bubbles that are smaller than the ejection ink droplets, and various problems may arise before the nozzles become completely blocked.
Even supposing that the aforementioned data is the most extreme data, if the waveform data in the case of air bubbles which are smaller than the ejected ink droplets is estimated from the indicated data, then although there is thought to be virtually no difference during normal operation, it is difficult to determine accurately a state of ejection failure or a state preceding ejection failure in the method described in Japanese Patent Application Publication No. 11-99646. Hence, further development is required in this respect.
The method described in Japanese Patent Application Publication No. 7-132592 has the object of correcting and controlling the drive waveform with respect to causes of fluctuation, but it does not seek to measure or determine ejection failure phenomena accurately. Furthermore, in practice, in the method described in Japanese Patent Application Publication No. 7-132592, it is stated that a corrected waveform may be determined on the basis of any one or any one part of a plurality of liquid droplet ejection apparatuses, this waveform being applied to the other liquid droplet ejection apparatuses. It is also stated that, in setting the measurement voltage waveform, the voltage does not necessarily have to reach a voltage value of a level that enables ink droplets to be ejected. However, there is no specific description regarding the desirable type of waveform.
Moreover, the method according to Japanese Patent No. 3495898 describes setting the pulse width applied to the head to the minimum pulse width that allows stable ink ejection, as a beneficial head driving method when ejection failures in a head that ejects ink by means of thermal energy are determined. However, this method cannot be regarded as an optimal method for use with a piezoelectric type of head.
In this way, in the determination of ejection failures in the related liquid ejection apparatuses described above, there are still problems in respect of determination accuracy and there remains scope for further improvement. Furthermore, it is desirable that restoration (maintenance) work be carried out efficiently in cases where an ejection failure has been detected.